The present invention relates to a resistance-film type touch panel for used in a liquid crystal display device, and a liquid crystal display device equipped with the same, and more specifically, to a touch panel incorporated between an observation-side polarizer plate and a liquid crystal cell of a liquid crystals display device, and a touch panel-integrated liquid crystal display device.
These days, the liquid crystal display element is drawing an attention as an image display element, and applied to a portable electronic notebook, or other information terminals.
As an input device for these portable information terminals, a resistance-film type touch panel is employed which is placed on a liquid crystal cell via a spacer.
The resistance-film type touch panel has a structure in which a movable (flexible) transparent electrode plate composed of a flexible transparent substrate provided with a transparent electrode layer, and a fixed (rigid) transparent electrode composed of a rigid transparent substrate also provided with a transparent electrode layer are arranged space apart from each other via a spacer such that their transparent electrode layers face each other. The flexible electrode plate yields by pressing applied from the flexible electrode plate with a pen or a finger and its transparent electrode layer is brought into point contact with the transparent electrode layer of the rigid electrode plate, thus the two electrodes are electrically connected with each other, thereby detecting the contact position (input position).
For the substrate of the flexible electrode plate of the touch panel described above, a thin glass or a plastic film has been used. As the plastic film, a polyethylene telephthalate (PET) film is generally used, and a transparent electrode layer made of indium-tin oxide (ITO) is formed thereon via an undercoat layer.
Meanwhile, as the use of the touch panel expands recently, there are demands of improving the display quality such as an improvement of the display contrast and the prevention of the external light reflection. In order to meet the demands, there has been an attempt of integrating a touch panel and a liquid crystal cell. For Example, as disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 3-121523 and 10-48625, the touch panel-integrated liquid crystal display device has a structure in which a touch panel is provided between an observation side (upper side) polarizer plate and a liquid crystal cell, and the flexible electrode plate of the touch panel is fixed to the lower surface of the observation-side polarizer plate of the liquid display device.
With a touch panel-integrated liquid crystal display device of such structure, the contrast of the entire display device can be improved, and further the reflection light from the inside of the touch panel can be avoided by imparting an appropriate retardation to the plastic film which constitutes the flexible substrate of the flexible electrode plate, or attaching another retardation film thereon.
Here, it should be noted that the plastic film used as the substrate for the flexible electrode plate of the touch panel is required to have uniform optical properties since the optical properties thereof has influence on the light polarization state. Especially, such a plastic film is required to have an extremely low retardation (uniform optical isotropy) or a uniform retardation (uniform optical anisotropy). A PET film, which has conventionally been used, is a crystalline plastic material, and partially crystallizes within the film. Consequently, the molecular orientation is not uniform, and therefore the retardation is widely varied. Under these circumstances, there is an attempt of using, in place of a PET film, a film obtained by subjecting amorphous plastic such as polycarbonate or polyarylate to solution casting.
However, the inventors of the present invention have found for the first time that when conventionally used polycarbonate or polyarylate is used as the substrate for the flexible electrode plate of the touch panel, and it is fixed to the observation-side polarizer film of the liquid crystal display device, the optical properties of the film substrate greatly vary under hot and humid environments. The variation of the optical properties of the film substrate under hot and humid environments is considered to be due to the fact that the polarizer plate, which has an inferior humidity and heat resistance, deforms when the touch panel-integrated liquid crystal display device is placed under a humid and hot environment during use, resulting in that stresses are applied to the flexible film substrate fixed to the polarizer, leading to the change in the optical characteristics of the flexible film substrate. This phenomenon has been found to be due to the circumstances special to the structure of the touch panel, in which the film substrate on which the polarizer plate is fixed, is a plastic film which has a flexibility, and the plastic film is fixed only at its periphery to a rigid substrate such as glass, thus being easily deformed.
Such a variation itself in the optical properties of the film substrate, caused by the stress generated during use will be suppressed by increasing the thickness of the film substrate. However, when the film substrate is thickened, the rigidity of the film substrate increases to require an excessive amount of pressing force in order to detect an input position with the touch panel, that is, in order to deform the flexible electrode plate, to ultimately lose the flexibility of the film substrate, thereby making it impossible to detect the input position.
Further, an amorphous acryl-based plastic or amorphous olefin-based plastic itself, for example, has optical properties which vary little, and therefore it is considered to use these plastic materials as the flexible film substrate. However, it has been found that these plastic materials have a low heat resistance, and therefore they may deform at the temperature of processing the touch panel. They also have a low polarity, and thus is difficult to provide good adhesion with a transparent electrode layer or various coating layers, formed on its surface. It has also been found that it is difficult to intentionally impart a retardation thereto by use of means such as drawing.
Therefore, an object of the present invention is to provide a resistance-film type touch panel capable of substantially maintaining the initial optical properties even if the polarizer plate is deformed as the panel is placed under an environment of a high temperature and a high humidity while being used.
Further, another object of the present invention is to provide a resistance-film type touch panel which does not substantially exhibit the change in color tone even if the panel is placed under an environment of a high temperature and a high humidity while being used.
Still another object of the present invention is to provide a resistance-film type touch panel capable of preventing the reflection of external light.
Still another object of the present invention is to provide a liquid crystal display device in which a touch panel of the present invention is incorporated.
In order to achieve the above-described objects, the inventors of the present invention conducted intensive researches and studies and have found that a film made of an amorphous plastic material having an optical elasticity coefficient in a certain range does not substantially vary its optical properties even if a stress is applied due to the deformation of the polarizer plate, caused under a high-temperature and high humidity environment, and further is capable of providing an excellent film substrate for a flexible electrode plate, to which a uniform optical anisotropy can easily be imparted when desired. The present invention is based on these findings.
Thus, the present invention provides a touch panel for use in a liquid crystal display device, comprising: a rigid electrode plate including a first substrate which is transparent and relatively rigid, and has a first main surface and a second main surface, and a first transparent electrically conductive layer provided on the second main surface of the first substrate; a flexible electrode plate including a second substrate which is transparent and flexible, and has a first main surface and a second main surface, and a second transparent electrically conductive layer provided on the first main surface of the second substrate, the flexible electrode plate arranged to be apart from a rigid electrode plate such that the second transparent conductive layer faces the first transparent conductive layer; and a polarizer plate fixed on the second main surface of the second substrate; wherein the flexible electrode has a flexibility sufficient to yield by a pressing force applied pointwise on the polarizer plate, thereby to bring the second transparent conductive layer into point contact with the first transparent conductive layer, the second substrate is made of a transparent film of a transparent amorphous plastic material having a glass transition temperature of 100xc2x0 C. or higher, and the transparent film has a relative optical elasticity coefficient Cr value of 0.2 to 0.8 defined by an equation:
Cr=C/Cpcxe2x80x83xe2x80x83(1)
where C represents an optical elasticity coefficient of the transparent film and Cpc represents an optical elasticity coefficient of a polycarbonate film derived from bisphenol A).
The present invention also provides a liquid crystal display device equipped with a touch panel of the present invention.
Further characteristics of the present invention will become apparent from the detailed descriptions provided below with reference to the accompanying drawing, and are recited in the claims attached hereto.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.